1. Field of the Invention
The present invention relates to a power factor correct current resonance converter, and more particularly, to a power factor correct current resonance converter formed by cascade-connecting a PFC (Power Factor Correct) converter circuit to a current resonance converter circuit.
2. Background Art
A DC-DC converter is a power source apparatus that steps up or down a direct-current input voltage to provide a certain direct-current output voltage. A resonance (or resonant) converter circuit is known as a type of the DC-DC converter. The resonance converter circuit has the property of being capable of providing favorable output load characteristics when the direct-current input voltage is not changed. Thus, in using a commercial alternating-current power source as an input power source for the resonance converter circuit, a power factor correct converter circuit is placed in the previous stage to stabilize the input voltage in a commonly used configuration (See, for example, Patent Documents 1 and 2). A specific configuration example of such a DC-DC converter of two-stage configuration is shown below.
FIG. 13 is a circuit diagram showing an example of a conventional DC-DC converter.
The DC-DC converter includes a current resonance converter circuit 100 which receives a direct-current voltage Vs and outputs a direct-current voltage Vo and a power factor correct converter circuit 110 which outputs the direct-current voltage Vs from a voltage Vd provided by rectifying a commercial alternating-current power source 120 with a diode bridge DB.
In the current resonance converter circuit 100, a series circuit including two switches Q1 and Q2 is connected in parallel to the direct-current voltage Vs, and a control circuit CONT1 is connected to each of the gates of the switches Q1 and Q2. The current resonance converter circuit 100 also includes a resonance capacitor Cr and a resonance inductor Lr connected in series, and the resonance inductor Lr is connected to a winding P1 on a primary side of a transformer T. An equivalent circuit of the winding P1 of the transformer T is connected in parallel to an exciting inductance Lm. Windings S1 and S2 on a secondary side of the transformer T are connected to a rectifying/smoothing circuit which is formed of diodes D1 and D2 and a smoothing capacitor Co to output the voltage Vo. The smoothing capacitor Co is connected in parallel to a feedback circuit 102. The feedback circuit 102 includes a photocoupler PCe (light-emitting diode) which emits light depending on variations in the voltage Vo at the output and a photocoupler PCr (phototransistor) which receives the emitted light such that the phototransistor of the photocoupler PCr on the light-receiving side is connected to the control circuit CONT1. The control circuit CONT1 controls the on-times or the switching frequencies of the two switches Q1 and Q2 based on the voltage Vo at the output to stabilize the voltage Vo at the output.
The power factor correct converter circuit 110 includes a choke coil Lp, a diode Dp1, a switch Q3, a smoothing capacitor Cs, a resistance Rs for current detection, and a control circuit CONT2. The control circuit CONT2 detects the voltage Vs across the smoothing capacitor Cs and detects a current passing through the choke coil Lp with the detection resistance Rs, controls the switch Q3 based on these detected values, and provides the input current of sinusoidal wave form to improve the power factor.
In the conventional DC-DC converter, the current resonance converter circuit 100 and the power factor correct converter circuit 110 include the control circuits CONT1 and CONT2, respectively, and the control circuits CONT1 and CONT2 operate independently. For this reason, the current resonance converter circuit 100 and the power factor correct converter circuit 110 employ different control methods and different switching frequencies.